The present invention relates to fuel injectors suitably used for injecting fuel to an engine, for instance, an automobile engine.
Generally, fuel injectors used for automobile engines include a tubular casing having an axial fuel passage and made of magnetic metal. A valve seat is disposed at one end of the fuel passage and has a fuel outlet. A core cylinder is disposed within the fuel passage in axially spaced relation to the valve seat. A valve element is axially moveably disposed within the fuel passage. An electromagnetic actuator is provided for forcing the valve element to an open position upon being energized. In the open position, the valve element is out of contact with the valve seat to allow fluid to be injected through the fuel outlet.
Japanese Patent Application First Publication No. 11-6467 discloses such an electromagnetically operated fuel injector. The fuel injector of this related art also includes a casing, a core cylinder axially opposed to the valve element with an axial air gap, and a non-magnetic joint disposed between the casing and the core cylinder. When the electromagnetic actuator is energized, a magnetic field extends to the core cylinder and the valve element via the axial air gap, so that the valve element is attracted by the core cylinder and moved to the open position. The non-magnetic joint suppresses a short-cut of the magnetic field which will be caused between the casing and the core cylinder. If the short-cut is caused, the magnetic field will form a closed magnetic circuit without passing through the axial air gap between the core cylinder and the valve element. This will cause reduction of the magnetic force acting on the valve element and the core cylinder.
Japanese Patent Application First Publication No. 2000-8990 also discloses an electromagnetically operated fuel injector of the same type as described above. The fuel injector includes a casing formed by a metal pipe, and an annular non-magnetic portion disposed in an axial-middle position of the casing. Upon energizing the electromagnetic actuator, the annular non-magnetic portion prevents the short-cut of the magnetic field. The annular non-magnetic portion is formed by subjecting the axial-middle portion of the metal pipe to heat treatment, for instance, induction heating.
Japanese Patent Application First Publication No. 2001-27169 discloses such an electromagnetically operated fuel injector of the same type as described above. The fuel injector includes a tubular casing having a fuel outlet at one axial end portion thereof, a resin cover covering an opposite axial end portion of the casing, a seal disposed on near the one axial end portion of the casing, and a resin protector for protecting the one axial end portion of the casing and the seal. Upon manufacturing the fuel injector, the parts such as a valve seat, a valve element, a core cylinder and an electromagnetic actuator are mounted to the casing, and the resin cover is formed by injection molding. An axial air gap (a valve lift amount) between the valve element and the core cylinder is adjusted using a tool. After the adjustment work, the resin protector previously molded is mounted onto the one axial end portion of the casing together with the seal.
Upon manufacturing the fuel injector of Japanese Patent Application First Publication No. 11-6467 described above, a forming work of the non-magnetic joint and the core cylinder and an assembly work thereof necessitate relatively much time and effort. This is because the non-magnetic joint and the core cylinder have engaging portions engageable with each other upon assembling, which complicate the shapes of the joint and the core cylinder. This will deteriorate the productivity and increase the number of parts, leading to a complicated structure of the fuel injector and reduction in reliability thereof. In the fuel injector of Japanese Patent Application First Publication No. 2000-8990 described above, the casing tends to suffer from thermal deformation which will be caused by the heat treatment. This will cause slight distortion and warping in the casing, causing erroneous assembly of the parts such as the valve element, the core cylinder and the electromagnetic actuator. Also, it is likely that, upon operating the fuel injector, the valve element fails to smoothly move within the casing due to the distortion and warping of the casing. Further, upon manufacturing the fuel injector of Japanese Patent Application First Publication No. 2001-27169 described above, the protector must be separately molded and be mounted onto the casing after the adjustment of the axial air gap between the valve element and the core cylinder for facilitating the adjustment work. Much time and effort are required to form the protector in a molding process separated from the assembly line, and then mount the molded protector onto the casing. This leads to deterioration in productivity of the fuel injector.
The present invention contemplates to solve the above-described disadvantages or problems of the related arts. Specifically, it is an object of the present invention to provide a fuel injector using a partially magnetically interrupted casing, which has a simple structure with a reduced number of parts and is improved in productivity and reliability. Also, it is an object of the present invention to provide a method of manufacturing the fuel injector, in which the partially magnetically interrupted casing is readily formed with high accuracy by general machining, and the resin protector is formed and mounted to the casing in a simple manufacturing line of the fuel injector, serving for reducing the number of parts and improving the working efficiency upon assembling.
According to one aspect of the present invention, there is provided a fuel injector, comprising:
a tubular casing defining an axial fuel passage;
a valve seat element disposed within said axial fuel passage, said valve seat element defining a fuel outlet communicated with said axial fuel passage;
a valve element axially moveable within said axial fuel passage between an open position where said valve element is out of contact with said valve seat element to allow fluid communication between said axial fuel passage and said fuel outlet and a closed position where said valve element is in contact with said valve seat element to block the fluid communication;
a core cylinder axially opposed to said valve element with an axial air gap;
a spring biasing said valve element toward the closed position, said spring being disposed within said axial fuel passage; and
an electromagnetic actuator cooperating with said casing, said valve element and said core cylinder to form a magnetic field forcing said valve element to the open position against said spring upon being energized,
said casing being formed with a reluctance portion producing an increased magnetic reluctance and allowing the magnetic field to pass through the axial air gap between said valve element and said core cylinder, said reluctance portion having a reduced radial thickness and an axial length extending over the axial air gap.
According to a further aspect of the present invention, there is provided a method of manufacturing a fuel injector, the fuel injector including a tubular casing having an axial fuel passage, a valve seat element disposed within the fuel passage at one axial end portion of the casing, an electromagnetic actuator disposed on the casing, a core cylinder axially spaced from the valve seat element, a valve element axially moveable between the valve seat element and the core cylinder and opposed to the core cylinder with an axial air gap, the casing cooperating with the core cylinder and the valve element to form a magnetic field upon energizing the electromagnetic actuator, the casing including a reluctance portion which has a reduced radial thickness and an axial length extending over the axial air gap, the method comprising:
forming an annular groove on an entire circumferential surface of a pipe made of magnetic material to provide the tubular casing formed with the reluctance portion;
fixing the valve seat element into an inner circumferential surface of the one axial end portion of the casing;
fixing the electromagnetic actuator onto an outer circumferential surface of the casing; and
mounting the valve element and the core cylinder into the casing so as to be opposed to each other with the axial air gap to provide the fuel injector.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.